The plasma membrane (outer membrane) of intact eukaryotic cells is characterized by a highly organized structure. This high level of organization is determined, among others, by the molecular structure of the specific lipids constituting the membrane; the ratio between the various lipid species from which the membrane is composed; the distribution of the phospholipids between the outer and inner leaflets of the membrane; and by the membrane protein constituents.
While maintenance of the high level of membrane organization is fundamental to normal cell physiology, substantial perturbations and alterations of the normal organization of the cell plasma membrane (PNOM) occur in numerous physiological and pathological conditions, and are characterizing a plurality of diseases (Martin, S., et al., 1995). Such alterations and perturbations may be evident both at the morphological level (membrane blebbing observed in cells undergoing apoptosis) and at the molecular level. The scope of perturbations accompanying either cell activation, cell disease or cell death is not fully elucidated. They include, among others, scrambling and redistribution of the membrane phospholipids, with movement to the cell surface of aminophsopholipids, mainly phosphatidylserine (PS) and phosphatidylethanolamine (PE), which are normally restricted almost entirely to the inner leaflet of the membrane bilayer, and reciprocal movement of sphingomyelin (SM) and phosphatidylcholine (PC) from the outer leaflet to the inner leaflet of the membrane (Sims, P. J., et al., 2001). This redistribution is referred herein as loss of cell membrane lipid asymmetry (CMLA).
These alterations play an indispensable role in making the cell surface a catalytic platform for the assembly of several clotting factor complexes, such as the tenase and prothrombinase protein complexes (Bevers, E. M., et al., 1999). Thus, platelets undergo PNOM upon activation, and these alterations constitute an important factor in normal blood coagulation, as well as in the initiation and/or propagation of abnormal, excessive blood clotting in numerous disorders. These disorders include, among others, arterial or venous thrombosis or thrombo-embolism [e.g., cerebral stroke, myocardial infarction, deep vein thrombosis (DVT), disseminated intravascular coagulation (DIC), thrombotic thrombocytopenic purpura, etc.]; unstable atherosclerotic plaques, sickle cell disease; beta-thalassemia; anti-phospholipid antibody syndrome; among others in systemic lupus erythematosus (SLE); disorders associated with shedding of membrane microparticles, e.g., neurological dysfunction in association with cardiopulmonary bypass.
Apoptosis is another major situation in which alterations/perturbations of cell membrane take place (Bratton, D. L., et al., 1997). Apoptosis is an intrinsic program of cell self-destruction or “suicide”, which is inherent in every eukaryotic cell. In response to a triggering stimulus, cells undergo a highly characteristic cascade of events of cell shrinkage, blebbing of cell membranes, chromatin condensation and fragmentation, culminating in cell conversion to clusters of membrane-bound particles (apoptotic bodies), which are thereafter engulfed by macrophages (Bursch, W., et al., 1992). PNOM is a universal phenomenon in apoptosis, it occurs early in the apoptotic cascade, probably at the point of cell commitment to the death process, and has also been shown to be an important factor in the recognition and removal of apoptotic cells by macrophages (Van den Eijnde, S. M., et al., 1997).
A strong correlation has been recently drawn between PNOM and potent procoagulant activity of apoptotic cells (Bombeli, T., et al., 1997). PNOM in apoptotic endothelial cells, such as in atherosclerotic plaques (Mallat, Z., et al., 1997), probably plays an important role in the pathogenesis of thrombotic vascular disorders. PNOM is also a feature of inflammatory cells (i.e., lymphocytes, macrophages), activated by various triggers.
Since apoptosis, thrombosis or inflammation have an important role in the majority of medical disorders, it is desirable to have tools for detection of these biological processes. Compounds for selective binding to PNOM membranes, potentially also performing subsequent entry into these cells having such PNOM membranes, may therefore serve as an important tool for detecting and targeting of cells undergoing damage or death process, especially by apoptosis, or undergoing activation. In the clinical context, detection of binding to said membranes may be useful in the diagnosis of disease, in monitoring course or progression of a disease, or in monitoring the effect of various therapeutic approaches utilized to alter disease course.